Transmission line



April 29, 1952 M. c. PEAsE TRANSMISSION Lima:

Filed Aug. 3o. 1945 1N VEN TOR.

MARSHALL C. PEASE Til Q L k\\ o Y V l w Q\ ATTORNEY Patented Apr. 29, 1952 TRANSMISSION LINE Marshall C. Pease, Boston, Mass., assigner to the United States of America as represented by the Secretary of War Application August-3i), 1945, Serial No. 613,612

s claims. (01.'178-44) This invention relates generally to the transmission of radio frequency energy. More particularly lit relates to means for preventing changes in characteristic impedance arising from the use of dielectric supports for the inner conductor in coaxial lines.

In the construction of coaxial transmission lines it is necessary to provide supports spaced along the line to maintain the position of the inner conductor in the center of the outer conductor. If there is any deviation from the proper position, the` line will not have a fixed, known, calculable characteristic impedance. Indeed, over a long line the deviation may be so great.

that the conductors come close enough together to causing arcing over or they may even touch causingfa direct short circuit. v Since transmission lines, carrying any appre-' Several methods for supporting inner conducfv tors of concentric lines have been in use in the past ofl which three are so generally usedl` that theymay be considered as the only ones of prac-f` y tical importance.

The Yfirst of these methods is to use a continuous solid dielectric which completely lls the space between the conductors throughoutthe length of the line. While this gives complete support to the inner conductor, it has severalseri` ous disadvantages. The transmission of energy along the line is delayed to an extent that may be undesirable when a solid dielectric is used instead of air. Theiuse of a continuous solid dielectric support creates a problem when it is necessary to connect the inner conductor to some other type of conductor because of the necessity of connecting a'solid dielectric insulated conductor to an air insulated conductor with the resulting Vabrup change in characteristic impedance.

A second method is to support the inner conductor at intervals along its length by one-quarter wavelength coaxial stubs which may be manufacturedfas an integral portion of the line.. The outstanding disadvantage of this type of support is that the coaxial stub must be designed for 'operation at a predetermined frequency, and, if it is desired to operate the line for any frequency other fthan that one, the efficiency drops off very rapidly as departure from the designed frequency increases. The result is that for practical use such lines are limited to operation over a narrow band of frequencies. Further, for relatively low operating frequencies, the length of the stub becomes inordinately great.

The third method in common use is to use circular dielectric discs fitting snugly in the outer conductor and having a central hole fitting snugly over the inner conductor. These discs, commonly referred to in the art as beads, are spaced along the line and held in position by cementing or other suitable means. The disadvantage of this type of a line is that it does not have uniform characteristic impedance throughout its length. The portions of such a line between the beads where the only dielectric is air havef-a surge impedance which differs from the impedance at the small sections where the beads are located, since the impedance is determined in part by the dielectric constant of thematerial between the-conductors. A yrnodiiication of this type of line is to fill the entire length of the line with beads. This is, in eifect, the same as the use of a solid dielectric and has all the disadvantages of that type of line. In addition, when the line is flexed, trouble is encountered 'by reason of the small wedge shaped areas between beads at the bent portions.

It is an object of the present invention toprovide a coaxial transmission line in whichthe coaxial l'center conductor isv rigidly and accurately supported in the centerof the outer conductor by means which permit broad frequency band operation and which will not change the characteristic impedance frmpoint to point along the line.

It is a further object of this invention to permit the line to retain air as its dielectric over the principal portion of its length and to permit the design of a transmission line having any desired proportion of air insulated length.

It is also an object of this invention to devise a supported coaxial line which will permit easy and quick assemblyv closely following standard coaxial line construction. M

Generally, this invention comprises a coaxial transmission line with dielectric centering vsup-- ports in the form of discs which just fit inside the outer conductor and which have central holes just large enough tokpermit the innerconductor to be threaded through them and with conductive spacers between each dielectric support. These spacers may be sleeves tting over the center conductor thus increasing its diameter or may be liners that just fit-within the outer conductor thus decreasing its inner diameter. The result of both types is to decrease the characteristic impedance of the spaces between the supports. By choice of proper thickness of these spacers this follows:

Referring to the embodimentrepresented in' Figs. 1 and 2, the transmission line comprises outer conductor Il) and inner'conductor El.' Inner conductor Il is supported and spaced from outer conductor IB by insulating beads I2 which are of a circumference to t snugly within outer conductor I and have central holes I 3 just large enough to lit over inner conductor II. Discs I2 arc-:spaced along the line at any desired distance. "Metallic spacer sleeves M are made to have an 'inner diameter to iit'over inner conductorv I I and 'a vlength to reach from one insulating bead I 2 to the next.

'The'outer diameter of sleeves I4 is chosen so that the characteristic impedance at the portions 'of the' line vwhere they occur is the same as the ,characteristic impedance at the portions'where beads I2 occur. Since the effect of putting sleeves I4 over inner conductor II is tc increase the diameter of that conductor, this may be determined The formula for the characteristic impedance :where .",Z is the characteristic impedance, a is the outer diameter of the innerconductor, bis'the inner diameter of the outer conductor, and K .is

.the dielectric constant of the material separating the conductors, K for air is unity. y

vIf thediameter, c, of the inner conductor in- ...creased .by placing the sleeve over it, issuch that kthe characteristic impedance of thelineis the .sameover its wholelength, then the. following l relationship exists:

Solving for c,

`-In."theembodiment shownin Figs. 3 and 4 in- "stead'of 'fitting conductive 'sleeves' around inner conductor II, a similar result is obtained by t- `ting conductive liner sections I5. inside of outer conductor I0. y'lhe effect of this is to reduce the inner diameter 'of cuter conductor ID and, again,

the thickness of liners I5 are chosen so that the 'characteristic impedance of the portions of the "linewhere the liners occur is thesame asthe portions where .insulating .beads I2 occur. In

. .this case nvvherel the symbols represent the same factors as before and d represents the inner diameter of liners I5. Solving for d,

The sleeve l-'I and the bead I2 at the right hand vextremities of Fig. 1 are shown displaced from their normal position in order to show the simple method by which these elements may be slipped over the inner conductor to construct the line. One liner section I5 and one bead I2 are similarly shown in Fig.. 3.

1t 'is'` obvious 4that connection can be made to a transmission line employing the present invention without the removal of dielectric insulation.

`Thsxisaccomplished by making the connection at a point along the line Where either a sleeve I4 or, in the alternate embodiment, a liner I5 is placed. The advantage of such a connection is that discontinuity caused by cleaning a portion of the center conductor of a line employing continuous dielectric throughout its length, is avoided.

Sections of line employing the principles of this invention can easily be joined by any appropriate mechanical means which preserve the inner diameter of the outer conductor and the outer diameter of the inner conductor in the region of thc connection.

it Will be obvious to those skilled in the art that any suitable dielectric material may be used for supports I2 and any appropriate conducting material for sleeves Ill or liners I5. In order to reduce contact resistance, it is recommended that. where possible, the conductive material used for these parts be the same as that used for the conductors of the line. Further, the geometric shape of the coaxial line can be varied from thathaving a circular cross-section so long as the basic principles of this yinventicm are followed.

It will be seen that a coaxial line constructed in accordance with the teachings of the present invention will maintain -accurate and rigid centering oi the inner conductor, Will operate over a very wide band of radio frequencies, and will have a uniform value of characteristic impedance from one end to the other.

While there has been here described what Yis at present considered to be the'preferred embodiment of this invention, it will be obvious to .those skilled in the art that various changes and modincations may. be made therein without departing from the scope of the invention.

What is claimed is:

1. In a coaxial transmission line having an inner conductor concentric with an outer conductor, dielectric support mea-ns disposed between said inner and .outer conductorsat spaced inten vals along said inner conductors, said .dielectric support means serving to maintain saidinner conductor in concentric relationship with said outer conductor, the impedance of said line at each of said dielectric support means differing from that of the unsupported remainder of, said line, and a plurality of conductive section means disposed between said dielectric support means and slidably in contact with one of said conductors, .saidfsection means being 'concentric with Ysaid conductors, each 'ofsaid condu'ctive'section pedance of said line at said dielectric support means, resulting in a coaxial line having a uniform characteristic impedance despite the lpresence of said dielectric support means.

2. In a coaxial transmission line having an inner conductor concentric with an outer conductor, said conductors having given uniform diameters, dielectric support means disposed between said inner and outer conductors at spaced intervals along said inner conductor, said dielectric support means serving to maintain said inner conductor in concentric relationship with said outer conductor, the impedance of said line at each of said dielectric support means diiering from that of the unsupported remainder of said line, and a plurality of conductive sleeve means disposed between said dielectric support means and slidably in contact with the inner one of said conductors, said sleeve means being concentric with said inner conductor, each of said conductive sleeve means having an outer diameter such that the electrically effective outer diameter of said inner conductor, between said dielectric support means, is made equal to the original outer diameter of said inner conductor multiplied by a factor equal to the square root of the dielectric constant of said dielectric support means, the impedance of said line at all points between lsaid dielectric support means then being equal to the impedance of said line at said dielectric support means, resulting in a -coaxial line having a uniform characteristic impedance despite the presence of said dielectric support means.

3. In a coaxial transmission line having an inner conductor concentric with an outer conductor, said conductors having given uniform diameters, dielectric support means disposed between said inner and outer conductors at spaced intervals along said inner conductor, said dielectric support means serving to maintain said inner conductor vin concentric relationship with said outer conductor, the impedance of said line at each of said dielectric support means differing from that of the unsupported remainder of said line, and a plurality of conductive liner means disposed between said dielectric support means and slidably in contact with the outer one lof said conductors, said liner means being concentric with said outer conductor, each of said conductive liner 'means having an inner diameter such that the electrically effective inner diameter of said outer conductor, between said dielectric support means, is made equal to the original inner diameter of said outer conductor multiplied by a factor equal to the reciprocal of the square root of the dielectric constant of said dielectric support means, the impedance of said line at all points between said dielectric support means then being equal to the impedance of said line at said dielectric support means, resulting in a coaxial line having a uniform characteristic impedance despite the presence -of said dielectric support means.

MARSHALL C, PEASE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,403,252 Wheeler July 2, 1946 2,437,482 Salisbury Mar. 9, 1948 

